1. Field of the Invention
The present invention relates to a motor, for example, used for slewing in a construction machine.
2. Description of the Background Art
There is conventionally known a motor used for a construction machine or the like, the machine including a rotor, a stator with a coil that forms a magnetic field for rotating the rotor, and a stator frame that accommodates the stator. This type of motor can form magnetic field by current applied to the coil. However, in the case where comparatively large current is applied to the coil to thereby remarkably generate heat, it is required to dissipate the generated heat efficiently to the exterior of the stator frame to cool the coil.
As means for cooling such a coil, Japanese Patent Application Publication No. 2007-143245 discloses using a cooling resin including fillers exhibiting high thermal conductivity. Specifically, this document discloses a motor having a rotor that includes a permanent magnet, a stator that includes a coil for rotating the rotor, and a motor frame (a stator frame) that accommodates and holds the rotor and the stator, wherein the cooling resin is charged into the interior of the motor frame. The cooling resin is disposed so as to be interposed between an outer surface of the coil and an inner surface of the motor frame to function to advance thermal conduction from the coil to the motor frame.
In this conventional motor, however, as will be described below, it is difficult to determine a thickness of the resin charged into the stator frame. Although the resin includes fillers exhibiting high thermal conductivity, an overall thermal conductivity of the resin is lower than that of a material (typically a metal) constituting the stator frame, and therefore the thickness of the resin is required to be possibly reduced in order to advance heat radiation from the coil. On the other hand, the reduction in the thickness causes the resin to be easily cracked by external force due to vibration of the rotor including an output shaft, and so on. The cracks may generate fragments of the resin, the fragments having a possibility of being caught in the rotor to cause an operation defect therein. Besides, increasing content of the fillers to secure high thermal conductivity without reducing the thickness of the resin involves a large cost increase. These problems are particularly evident in a type of motor where large current is applied to the coil and a large mechanical load is exerted on the rotor, for example, as a motor for slewing an upper slewing structure of a construction machine.